Dye stripping process employing stabilized hydroxyalkane sulfinic acid baths



United States Patent ()fice Patented May 23, 1961 DYE STRIPPING PROCESS EMPLOYING STABI- %TAZED HYDROXYALKANE SULFINIC ACID Alfons Janson, Ludwigshafen (Rhine)-ppau, and Johannes Conrad and Karl Morweiser, Ludwigshafen (Rhine), Germany, assignors to Badische Aniliu- & Soda-Fabrik Aktiengesellschaft, Ludwigshafen (Rhine), Germany No Drawing. Filed Dec. 2, 1958, Ser. No. 777,567

Claims priority, application Germany Dec. 6, 1957 8 Claims. (Cl. 8-102) The present invention relates to the stabilization of acid stripping baths which contain as reducing agents salts of aliphatic sulfinic acids, by means of low molecular weight aliphatic aldehydes or compounds which split off aldehydes under the conditions of the stripping process, and to solid mixtures for the production of such stripping baths in which the reducing agent is stabilized in the said manner.

In order to decolor textile goods it is possible in many cases, for example when it is a question of dyeings obtained with azo or vat dyestuffs, to use aqueous solutions of reducing agents which convert the dyestulis into uncolored water-soluble substances which are easy to remove. For this working operation, which is known as stripping, salts of sulfinic acids have above all proved suitable. Especially the sodium, potassium, calcium and zinc salts of hydroxymethane and hydroxyethane sulfinic acids are known as stripping agents. However allied substances are equally suitable, for example the salts of the homologous hydroxysulfinic acids with up to 4 carbon atoms in the molecule and the salts of the condensation products of low molecular weight hydroxyalkane sulfinic acids with ammonia or ethylene diamine in which one or more sulfinic acid radicals are combined to nitrogen with the loss of their hydroxyl groups.

For stripping, aqueous baths containing said reducing agents are brough to a pH value of 3 to 6 by the addition of acid substances, for example formic acid. In this bath the dyed textile materials,'for example fibers, which are to be decolorized are heated for about one hour at 50 to 95 C. whereby a stripped material is obtained which is colorless or still weakly colored depending on the nature of the dye-stuff and the shade of color.

A considerable disadvantage of this known method resides in the fact that stripping baths of the said kind are quite unstable during the necessary prolonged heating. There are normally formed insoluble decomposition products of the sulfinic acids serving as reducing agents and these combine very firmly with the fibers and injure their properties. Especially, the textile material substantially loses its soft handle. The deposits cannot be removed subsequently from the textile fibers without damage thereto and present an extremely troublesome eflfect in redyeing. Moreover the decolored material acquires an unpleasant odor which adheres very firmly.

It is, therefore, an object of this invention to provide a method for stabilizing baths which are prepared from a salt of an aliphatic sulfinic acid as reducing agent.

Another object of this invention is to prevent the decomposition of the sulfinic acids or their salts when they are heated in an aqueous solution.

A further object is to prevent the formation of insoluble decomposition products of sulfinic acids on textile materials during the stripping process.

Still a further object is to provide a method for stripping, i.e. decolorizing, textile materials without impairing their soft handle.

An additional object is to provide a method for stripping textile materials which can be redyed after the stripping without difficulties.

Still a further object of the invention is to provide a stripping bath which may be exhausted considerably more than has hitherto been possible.

Another object is to provide a concentrated or solid stripping composition based upon sulfinic acid which has an improved stability against decomposition during the application in a more dilute and dissolved form.

These objects are achieved in accordance with the invention by incorporating in the concentrate or in the stripping bath low molecular weight saturated aliphatic monoor di-aldehydes or compounds which split oif those aldehydes under the conditions of the stripping process. These aldehydes and compounds splitting off aldehydes have an excellent stabilizing effect against the decomposition of sulfinic acids and their salts which are present in stripping baths. The aldehydes and compounds splitting ofi' aldehydes can, therefore, in accordance with this invention, be called stabilizers.

Low molecular weight saturated aliphatic monoor di-aldehydes in the scope of this invention are aldehydes which can be used in textile finishing processes. We prefer monoor di-aldehydes which have one or two carbon atoms in the molecule. The most suitable aldehydes, above all, are formaldehyde, acetaldehyde and glyoxal. Other aldehydes which may be used are, for example, propionaldehyde, butyraldehyde and suberic' aldehyde. By reason of their availability and the low boiling point we prefer the mono-and di-aldehydes with one or two carbon atoms in the molecule. Compounds which split off said aldehydes or form them under the conditions of the stripping process are, for example, paraformaldehyde, formaldehyde-urea condensation products containing from one to three methylol groups and formaldehyde-ammonia condensation products like hexamethylene tetramine. The latter is particularly efiective as a stabilizing agent within the scope of this invention. Mixtures of aldehydes and/or compounds which split off aldehydes may also be used.

There exist many compounds which split olf aldehydes, however, not all these compounds can be used as stabilizing agents in stripping baths. It is necessary that the compounds which shall act as stabilizers form a compound with an aldehyde group under the conditions of the stripping process. These conditions are known to those skilled in the art and, therefore, the choice of suitable stabilizing agents which are not aldehydes but which form aldehydes can be left to those skilled in the art.

As the stripping process used in the practice of our invention has long been known as such, it being the process which is commonly used for the purpose, we shall only illustrate it briefly by giving the major conditions under which it is carried out as well as a number of other technical data which are important for its definition: the working with an aqueous solution of an aliphatic sulfinic acid or its salt with an amount of about 0.01 to about 3% by weight of the sulfinic acid or its salt; the provision for a certain amount of undestroyed reducing agent to be still present at the end of the stripping process; the necessity of an individual choice of the amount of active substance in view of the material to be treated. Furthermore in this process the pH value may range between about 3 and about 6; the temperature during the stripping process may be from about 45 to about 100 C.; the time necessary for carrying out the stripping process may be from about 20 minutes to about 2 hours; the acids by which the pH value is adjusted may be inorganic or organic, for example formic acid, lactic acid, acetic acid, phosphoric acid, hydrochloric acid or sulfuric acid.

Hydroxyalkane sulfinic acids which are operable as reducing stripping agents are especially those which have 1 to 4 carbon atoms in the molecule, such as hydroxymethane sulfinic acid, hydroxyethane sulfinic acid, hydroxyisopropane sulfinic acid, alpha-hydroxybutane sulfinic acid. Suitable salts of these acids may have the equivalent amount of the cations sodium, potassium, calcium and zinc. Other suitable sulfinic acids are those which are obtained by the reaction of a hydroxyalkane sulfinic acid with ammonia or ethylene diamine,

Condensation takes place by this reaction between one or more molecules of the hydroxyalkane sulfinic acid and ammonia or ethylenediamine. In this way the hydroxylic group is replaced by an amino group which may have been condensed with further sulfinic acid molecules. The stripping baths can also contain mixtures of the above disclosed sulfinic acids or their derivatives. All these sulfinic acids or sulfinic acid derivatives can be designated by the term a sulfinic acid reducing agent.

It is pointed out that the invention is generally applicable to the stabilization of a sulfinic acid reducing agent and is not necessarily restricted to stripping baths and stripping processes to which the above data have been applied,

The stripping baths can also contain textile auxiliaries, such as dispersing agents for the stripped dyestulf. Dispersing agents which are especially suitable can be chosen from polyethylene glycol compounds, for example condensation products of high molecular aliphatic alcohols, amines or carboxylic acid amides with ethylene oxide, as, for example, the condensation product of 1 mol of octadecyl alcohol and 25 mols of ethylene oxide. Anionic dispersing agents, such as aliphatic-aromatic sulfonic acids salts, may also be added to the stripping baths. An example of such an additive is sodium dibutylnaphthalene sulfonate.

The stabilizers must be present at least in an amount which is sufiicient to provide a stabilizing effect on the reducing agent. Even small amounts of the stabilizer are effective. A preferred amount ranges from about 1 to 10% by weight of the stabilizer with reference to the reducing agent, although quantities of from 0.1 to 25% are generally useful.

The stabilizing substances may be added to the stripping baths at any time during their preparation. It is thus possible to add the aldehydes and substances yielding aldehydes to the finished baths immediately prior to use. Equally well, however, they may be incorporated with the reducing agent prior to the dissolution of the latter and the resultant mixtures used for the preparation of the stripping baths.

It is especially advantageous to mix 1 to 10% of hexamethylene tetramine with the sodium, potassium, calcium and zinc salts of hydroxyalkane sulfinic acids with 1 to 4 carbon atoms in the molecule or their condensation products with ammonia or ethylene diamine used as reducing agents. There is thus obtained an extremely stable and readily usable product which yields a stable stripping bath by simple dissolution in Water and acidification of the solution.

The stripping baths prepared according to one of the said methods decompose only very slightly even after long prolonged heating. Undesirable deposits and unpleasant odors in the baths and on the textile material to be decolored are thereby entirely avoided. Moreover the reducing baths may be exhausted considerably more strongly.

The dyed fibers which can be decolorized by stripping in accordance with this invention are those usually treated with sulfinic acid reducing agents in the art and may be loose fibers or processed fibers, i.e. fabrics or other kinds of textile material made from fibers. The fibers can contain wool, silk, polyamides, polyurethanes, polyacrylonitrile, polyesters and caseine derivatives.

The dyes contained in the fibers which are decolorized include vat dyestuffs, such as anthraquinone dyestuffs or indigo dyestuffs, azo dyestuffs and metal complex azo dyestulfs. While these dyestuflf groups are the most important to be stripped, there may be present on the fibers any other dyestutf which will be destroyed or at least solubilized by the reducing agent.

The following examples will further illustrate this invention but the invention is not restricted to these examples. The parts and percentages specified therein are parts and percentages by weight.

Example 1 A solution of 500 parts of the zinc salt of hydroxymethane sulfinic acid, 8 parts of 100% formaldehyde and 100 parts of formic acid in 100,000 parts of water, which has a pH value of 3, is heated to 60 C. After 30 minutes, 1.6% of the zinc salt of hydroxymethane sulfinic acid have decomposed and after 60 minutes 2.6%. The solution remains completely clear and odorless. Under the same conditions, but without the addition of formaldehyde, the reducing agent undergoes a decomposition of 30% in 30 minutes and 48% after 60 minutes. After about 10 minutes there occurs in the solution a strong turbidity from which a curdy precipitate forms which settles on the fibers when textile material is present.

By using 8 parts of hexamethylene tetramine instead of formaldehyde, the decomposition of the zinc salt of hydroxymethane sulfinic acid amounts to only 1.0% after 30 minutes and only 2.1% after 60 minutes.

Example 2 100 parts of the zinc salt of hydroxymethane sulfinic acid and 6 parts of water-soluble paraformaldehyde are dissolved in 10,000 parts of water at 60 C. and 20 parts of formic acid are added. The decomposition of the zinc salt of hydroxymethane sulfinic acid (which in the absence of the water-soluble paraformaldehyde reaches 3 8% after 30 minutes and 40% after 60 minutes) amounts to only 2% after 30 minutes and only 3.6% after 60 minutes. The solution remains completely clear and odorless. If it is used for stripping dyed woolen rags, a stripped material of very soft handle is obtained by heating to C.

Example 3 50 parts of the calcium salt of hydroxyethane sulfinic acid are dissolved in 10,000 parts of water with the addition of 5 parts of acetaldehyde and 39 parts of formic acid. The solution, which has a pH value of 3.2, is heated to 50 C. The decomposition of the hydroxyethane sulfinic acid amounts to 1.7% after 30 minutes and 6.8% after 60 minutes, the solution remaining completely clear and odorless. If the experiment is carried out without the addition of acetaldehyde, the solution becomes turbid even after a few minutes and acquires a very strong unpleasant odor. The decomposition of the hydroxyethane sulfinic acid proceeds very much more quickly. It is 85% after 30 minutes and 88% after 60 minutes.

If the acetaldehyde be replaced by 3.7 parts of hexamethylene tetramine, the pH value of the solution rises trivially to 3.4. The decomposition of the hydroxyeth Example 4 A mixture of 500 parts of the calcium salt of hydroxymethane sulfinic acid and 25 parts of hexamethylene tetramine is dissolved in 100,000 parts of water and 450 parts of formic acid. When heated to 60 C., 2.4% of the reducing agent decompose in 30 minutes and 4.2% in 60 minutes. Without the addition of hexamethylene tetramine, the decomposition amounts to 39% at 60 C. after 30 minutes and 47% after 60 minutes.

Example 5 A solution which contains in 200 parts of water: 1 part of formic acid and 2 parts of the calcium salt of a condensation product of 3 mols of hydroxymethane sulfinic acid and 1 mol of ammonia, begins to become cloudy and to decompose after about 15 seconds when heated to 60 C.

If however 0.3 part of glyoxal be added to such a solu tion prior to the heating, it remains clear and usable for stripping even after 40 minutes at 60 C.

Example 6 A stripping bath which contains in 1000 parts of water 2.2 parts of concentrated formic acid and 20 parts of the sodium salt of a condensation product of 4 mols of hydroxyethane sulfinic acid and 1 mol of ethylene diamine, begins to become turbid and to deposit flocculent decomposition products after about 1% minutes at 50 C. After 5 minutes, 62% of the reducing agent have been destroyed.

If there be added to the stripping bath, before it is heated, however, 0.7 part of formaldehyde (with reference to pure anhydrous aldehyde), only 2.5% are decomposed in 5 minutes at 50 0.; even after 40 minutes of continuous heating to 50 C., no turbidity or deposit is visible.

Example 7 200 parts of rags consisting of drab wool knit material are powerfully stirred for /2 hour at 60 to 70 C. and for another /2 'hour at 90 to 95 C. in a stripping bath consisting of 4,000 parts of water, 6 parts of the zinc salt of hydroxymethane sulfinic acid, 0.3 part of 30% formaldehyde and 7 parts of formic acid. The clear solution is then withdrawn and the stripped material is rinsed several times with water. A completely odorless material of very soft handle is obtained, which shows a uniform, weakly yellowish coloration.

When carrying out the process in a bath containing 7 parts of the zinc salt of hydroxymethane sulfinic acid and 7 parts of formic acid in 4,000 parts of water without the addition of formaldehyde, but under otherwise identical conditions, the stripped material has a most unpleasant smell of the decomposition products of the stripping agents and has a considerably harsher feel. Despite the larger amount of stripping agent the material is darker in color and less evenly stripped.

Example 8 96 parts of the zinc salt of hydroxymethane sulfinic acid are intimately mixed with 4 parts of hexamethylene tetramine and 10 parts of this mixture are dissolved in 4,000 parts of water. To this solution there are added 250 parts of rags of new worsted material and 9.3 parts of formic acid and the rags are agitated for 36 hour at 65 C. and for another /2 hour at 95 C. The material obtained after rinsing in the usual manner is well stripped, Weakly olive green, odorless and exhibits a nice and soft handle.

When using, instead of the mixture, specified above, 10 parts of the zinc salt of hydroxymethane sulfinic acid, there is observed an extremely unpleasant smell even during the stripping process, which cannot be removed even by repeated rinsing. The stripping effect is considerably inferior and the handle is not soft.

Example 9 200 parts of dark-colored wool union which have been carbonized and rinsed are treated as described in Example 8 at between 65 and C. with a solution of 6 parts of the calcium salt of hydroxymethane sulfinic acid, 0.36 part of water-soluble paraformaldehyde and 7 parts 'of formic acid in 4,000 parts of water and then rinsed well.

The stripped material is odorless, exhibits a very soft handle and is weakly pink in color.

When the same process is carried out in the absence of aqueous paraformaldehyde, the stripped material acquires an unpleasant smell which adheres strongly to the fibers. Large portions of the materialof green, blue and violet color are insufiiciently stripped.

Example 10 200 parts of blue knit goods are stripped in the way described above in a solution of 7 parts of the calcium salt of hydroxyethane sulfinic acid, 0.7 part of acetaldehyde and 10 parts of formic acid in 4,000 parts of water.

As compared with material which has been treated in a stripping bath which did not contain acetaldehyde, the material thus obtained is much more decolorized, more uniformly stripped, completely odorless and exhibits a very soft handle.

Solid mixtures which can be prepared as a concentrate and which are useful for the preparation of stripping baths by dissolving the concentrate in Water and adding an acid are set forth in the following table:

sulfinic acid reducing agent stabilizer sodiun salt of hydroxymethane sulfinic potasgium salt of hydroxymethane sulfinie c calcinim salt of hydroxymethane sulflnic paraformaldehyde hexa' aci fiuig acid and hydroxyisopropaue sulfinic aci corresponding salts of condensation products of said hydroxyalkane sulfinic acids with ammonia or ethylenediamine.

groups in the molecule).

It is possible to use one or more stabilizers together with the sulfinic acid reducing agents enumerated in the above table.

We claim:

1. A process for decolorizing dyed fibers by stripping which comprises treating the fibers at a temperature of about 45 C. to C. in an acidic stripping bath consisting essentially of an aqueous solution having a pH of about 3 to 6 and containing (a) about 0.1 to 3% by weight thereof of a reducing agent selected from the group consisting of a hydroxyalkane sulfinic acid with l to 4 carbon atoms in the molecule, the sodium, potassium, calcium and zinc salts of said acid and the condensation products of said acid with ammonia and ethylenediamine, and as a stabilizer for said reducing agent (b) about 0.1 to 25% by weight, with reference to said reducing agent, of a low molecular weight aldehyde.

2. A process as claimed in claim 1 wherein the quantity of said aldehyde is about 1 to 10% by weight, with reference to the reducing agent.

3. A process as claimed in claim 1 wherein the aldehyde is obtained in the aqueous bath from a member selected from the group consisting of formaldehyde, acetaldehyde, glyoxal, paraformaldehyde and hexamethylene tetramine.

4. A process as claimed in claim 3 wherein the quantity of the aldehyde stabilizer is about 1 to 10% by weight with reference to the reducing agent.

5. A process as claimed in claim 4 wherein the re ducing agent is the zinc salt of hydroxymethane sulfinic References Cited in the file of this patent acid and the stabilizer is hexamethylene tetramine.

6. A process as claimed in claim 4 wherein the re- UNITED STATES PATENTS ducing agent is the zinc salt of hydroxymethane sulfinic 1,594,670 Hollander Aug. 3, 1926 acid and the stabilizer is paraformaldehyde. 5 1,918,873 Schribner July 18, 1933 7. A process as claimed in claim 4 wherein the reducing 2,109,708 Pfister Mar. 1, 1938 agent is the calcium salt of hydroxyethane sulfinic acid 2,206,535 Lubs July 2, 1940 and the stabilizer is hexamethylene tetramine. 2,658,496 Harris et a1. Aug. 3, 1954 8. A process as claimed in claim 4 wherein the re- 2,914,374 Harris et a1 Nov. 24, 1959 ducing agent is the calcium salt of hydroxymethane sul- 10 finic acid and the stabilizer is hexamethylene tetramine. 

1. A PROCESS FOR DECOLORIZING DYED FIBERS BY STRIPPING WHICH COMPRISES TREATING THE FIBERS AT A TEMPERATURE OF ABOUT 45*C. TO 100*C. IN AN ACIDIC STRIPPING BATH CONSISTING ESSENTIALLY OF AN AQUEOUS SOLUTION HAVING A PH OF ABOUT 3 TO 6 AND CONTAINING (A) ABOUT 0.1 TO 3% BY WEIGHT THEREOF OF A REDUCING AGENT SELECTED FROM THE GROUP CONSISTING OF A HYDROXYALKANE SULFINIC ACID WITH 1 TO 4 CARBON ATOMS IN THE MOLECULE, THE SODIUM, POTASSIUM, CALCIUM AND ZINC SALTS OF SAID ACID AND THE CONDENSATION PRODUCTS OF SAID ACID WITH AMMONIA AND ETHYLENEDIAMINE, AND AS A STABILIZER FOR SAID REDUCING AGENT (B) ABOUT 0.1 TO 25% BY WEIGHT, WITH REFERENCE TO SAID REDUCING AGENT, OF A LOW MOLECULAR WEIGHT ALDEHYDE. 